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Nattokinase is an enzyme (a protein that speeds up biochemical reactions) that is extracted from a popular Japanese food called natto. Natto is boiled soybeans that have been fermented with a bacterium called Bacillus natto. 

Although nattokinase is promoted for use as a fibrinolytic and thrombolytic agent, clinical trials are lacking to support such use. There is also no substantial evidence that nattokinase lowers blood pressure in hypertension.

Nattokinase decreases the ability of blood to clot. This "thins the blood" and might protect against conditions caused by blood clots such as stroke, heart attack, and others.

Small, short-term trials evaluating nattokinase reported no adverse reactions. There is a theoretical risk of bleeding, based on a case report of acute cerebellar hemorrhage in a patient with a history of ischemic stroke. Thrombosis was reported after substitution of nattokinase for warfarin in a patient with a mechanical aortic valve. Rare cases of late-onset anaphylaxis with natto have been attributed to poly-gamma-glutamic acid, a product of the fermentation process that may be present in nattokinase supplements.